Polylactic acid blown film and method of manufacturing same

ABSTRACT

The invention provides polylactic acid (PLA) films substantially free of plasticizers and methods of manufacturing same by blown film process are provided. PLA films are environmentally degradable and have applications in, at least, packaging and labeling.

CLAIM OF PRIORITY

This application claims priority to U.S. application Nos. 60/605,151;60/609,827; and 60/672,515, filed Aug. 30, 2004, Sep. 15, 2004, and Apr.19, 2005, respectively, the disclosures of which are hereby incorporatedin their entirety by reference.

FIELD OF THE INVENTION

The present invention relates generally to films produced by blown filmprocesses. More specifically, the present invention relates to blownfilms comprising polylactic acid and methods of manufacturing same.

BACKGROUND OF THE INVENTION

In recent years, interest in compostable polymers, i.e. biopolymers, hasgreatly increased, and many companies have made efforts to market, forexample, packaging materials, hygiene products, sacks, and films withcompostable polymers. Polylactic acid (PLA), i.e., polylactide, orcondensation polymers which are based on lactic acid, are for manyreasons a very attractive group of biopolymers. Their principaldegradation product, lactic acid, is a product common in nature, it isnot toxic and is used widely in the food and pharmaceutical industries.

Films, particularly blown films, comprising PLA have proven difficult tomanufacture. Indeed, currently available PLA blown films require theaddition of additives such as plasticizers to enable their production.However, plasticizers are often undesirable for films with food-relatedapplications; they are costly; and they seldom, if at all, are asenvironmentally friendly as PLA itself. To circumvent these issues, somemanufacturers have resorted to manufacturing PLA film with castingmethodology (e.g., cast and tenner). However, casting methodologyproduces films with limited applications and can be 5 to 10 times morecostly than blown film processing.

Accordingly, there is a need for PLA blown films substantially free ofplasticizers. There is also a need for a method of manufacturing PLAfilms using blown film processing.

SUMMARY OF THE INVENTION

The foregoing needs are met, to an extent, by the present invention, inwhich PLA blown films, which are substantially free of plasticizers, andmethods of making and using same are provided.

In one embodiment of the instant invention, a method of making a PLAblown film is provided comprising (a) providing dry pellets of PLA, (b)melting the pellets to form a molten mass at a first desired viscosityvalue or range of values, (c) increasing the viscosity of the moltenmass to a second desired viscosity value or range of values, (d) forminga heated bubble from the resulting molten mass, and (e) collapsing thebubble to form a film, in which the PLA film is substantially free ofplasticizer. The pellets may be dried in a variety of different ways,including, for example, drying in a dehumidifying hopper. The meltingtemperature may include a wide range of temperatures. In one embodiment,the melting temperature is selected in the range from about 325° F. toabout 485° F., preferably from about 375° F. to about 425° F., and morepreferably at about 400° F. In some embodiments, the first viscosityvalue ranges from about 1,000 P to about 5,000 P at about 480° F. at anapparent shear rate of about 55 seconds⁻¹, preferably from about 2,000 Pto about 4,000 P at about 480° F. at an apparent shear rate of about 55seconds⁻¹, and more preferably about 3,000 P at about 480° F. and at anapparent shear rate of about 55 seconds⁻¹. In other embodiments, thesecond viscosity value may range from about 14,000 P to about 16,000 Pat about 375° F. at an apparent shear rate of about 55 seconds⁻¹,preferably from about 15,500 P to about 16,500 P at about 375° F. at anapparent shear rate of about 55 seconds⁻¹, and more preferably about15,000 P at about 375° F. and at an apparent shear rate of about 55seconds⁻¹. The viscosity increasing step may be carried out mostconveniently, but not exclusively, in a polymer cooling unit, and thestep of forming a heated bubble may include a stretching step, whichorients the film. Alternatively, the viscosity increasing step may becarried out by internal cooling of the die mandrel (by air or liquidfluid), controlling the temperature of the die (by heated or cooledliquid fluid), and/or by the addition of chemical viscosity enhancers,the latter being preferably added during or before the melting step.

In some embodiments of the disclosed invention, the method may furtherinclude annealing the film. The annealing step may be carried out at avariety of temperature ranges, such as a temperature ranging from about120° F. to about 285° F., preferably from about 140° F. to about 250° F.If desired, the bubble may be heated to a temperature greater than about100° F.

In another embodiment of the present invention, a method of pretreatinga PLA polymer blend to allow the manufacture of a blown film is providedcomprising (a) providing pellets of a PLA polymer blend, (b) melting thepellets to form a molten mass at a first desired viscosity value orrange of values, and (c) cooling the molten mass to a second desiredviscosity value or range of values. The second desired viscosity valuemay fall in the range of about two times to about ten times the firstdesired viscosity value, and preferably in the range of about four timesto about eight times the first desired viscosity value. The cooling maybe performed within a polymer cooling unit, as well as by air (orliquid) cooling the die inner mandrel through which the polymer film isblown, the use of viscosity enhancers as previously noted, cooling thedie by air or liquid, and combinations of these techniques.

In yet another embodiment of the present invention, a packaged good isprovided comprising a consumable item and a PLA blown film wrappedaround at least a portion of the consumable item. The consumable itemmay include batteries, cans, bottles, disposable lighters, pens anddecorative items, window boxes for food items (e.g., pastries, donuts),toys, and window envelopes. The PLA blown film may be clear, matte,translucent, or opaque, and may include a packaging label, optionallyprinted onto the film.

In still yet another embodiment of the present invention, a blown filmis provided, comprising a polylactic acid (PLA) polymer substantiallyfree of plasticizers. The PLA polymer may have a molecular weightaverage of about 100,000 to about 400,000. In some embodiments, thelactide concentration may optionally be lower than about 1 percent,and/or have a mesolactide level of about 10 percent to about 20 percent.

There has thus been outlined, rather broadly, certain embodiments of theinvention in order that the detailed description thereof herein may bebetter understood, and in order that the present contribution to the artmay be better appreciated. There are, of course, additional embodimentsof the invention, which may be described below and which fall within thescope of the subject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable both ofembodiments in addition to those expressly described and of beingpracticed and carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein, as well as theabstract, are for the purpose of description and illustration and shouldnot be regarded as limiting.

As such, those skilled in the art may appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an assembly for production of PLA blownfilms in accordance with one embodiment of the instant invention.

DETAILED DESCRIPTION

Detailed embodiments of the present invention are disclosed herein.However, it is to be understood that the disclosed embodiments aremerely exemplary of the present invention, which may be embodied invarious systems. Therefore, specific details disclosed herein are not tobe interpreted as limiting, but rather as basis for teaching one skilledin the art to variously practice the present invention.

Plastic films comprising polylactic acid films and a method of theirmanufacture are described. In particular, the present invention providesblown films comprising polylactic acid. The use of the term “film”includes not only film, but sheets as well.

In general, polymer nomenclature sometimes references polymers on thebasis of the monomer from which the polymer is made, and in otherinstances characterizes the polymer based upon the smallest repeatingunit found in the polymer. For example, the smallest repeating unit inpolylactide is lactic acid. However, in typical instances, commercialpolylactide will be manufactured by polymerization of lactide monomer,which is a dimer of lactic acid.

Since both lactic acid and lactide can achieve the same repeating unit,the general terms “polylactic acid,” “polylactide,” and “PLA” as usedherein refers to polymers having the repeating unit of formula I withoutany limitation as to how the polymer was made (e.g., from lactides,lactic acid and/or oligomers thereof), and without reference to thedegree of polymerization. As well, the terms are intended to includewithin their scope both polylactic acid based polymers and polylactidebased polymers, with the terms used interchangeably. That is, the terms“polylactic acid,” “polylactide,” and “PLA” are not intended to belimiting with respect to the manner in which the polymer is formed.

The polylactide used in the invention may be made from any lactide, orblends thereof, and by any polymerization process. A high molecularweight polymer can be produced by ring-opening polymerization fromlactic acid dimer (“lactide”). Lactic acid is optically active, and thusits dimer appears in four different forms: L,L-lactide; D,D-lactide;L,D-lactide (“mesolactide”); and a racemic mixture of L,L- andD,D-lactides. By polymerizing these either as pure compounds or atdifferent blend proportions, polymers are obtained which have differentstereochemical structures, which affects their resilience andcrystallinity and, consequently, also their mechanical and thermalproperties. The obtained polymers are usually hard and optically clear.

Lactide polymers can be in either an essentially amorphous form or in asemi-crystalline form. For various applications it may be desirable tohave the polymer in semi-crystalline form. Semi-crystalline films have,for example, superior heat resistance. As well, lactide polymer filmswith increased crystallinity generally degrade more slowly thanamorphous films under conditions of high humidity and heat which resultsin extended shelf life of the films.

A desirable range of compositions for semi-crystalline poly(lactide) isless than about 15 percent by weight meso-lactide and the remainingpercent by weight being either L-lactide or D-lactide, wherein at least85 percent comprises either all L- or all D-lactide isomer. A morepreferred composition contains less than about 12 percent by weightmeso-lactide and a most preferred composition has less than about 9percent by weight meso-lactide with the remainder being substantiallyall L-lactide and/or D-lactide.

Addition of even small amounts of meso-lactide to the polymerizationmixture can result in a polymer which is slower to crystallize thanpolymerization mixtures having lesser amounts of meso-lactide. Beyondabout 15 percent meso content the polymer remains essentially amorphousfollowing the annealing procedure discussed below.

While the instant invention can be suited with any PLA polymer of anygrade and composition, the concentration of D-lactide, in particular,can affect the physical properties of the resulting PLA polymer. By wayof example, increasing the percent of D-lactide in a polymer or apolymer blend reduces the capacity of the resulting polymer tocrystallize, which, in turn, increases undesirable degradation of thepolymer at higher temperatures. Said another way, lowering the weightpercent of D-lactide in a polymer composition increases the temperatureresistance of the polymer and hence the viscosity of the resulting meltat a given temperature as well.

It is desirable to provide a polylactide polymer composition for a givenapplication with desirable physical properties and desirable stabilityproperties. Various applications for the polylactide polymer compositionof the invention include the formation of fibers, coating paper,formation of films, injection molding, thermal forming, injection blowmolding, preparing filmed articles, and preparing twine. It should beappreciated that stability requirements for a particular application canbe more or less stringent than the stability requirements for anotherapplication. Thus, those skilled in the art will appreciate that thiswide latitude of applications must be accommodated by the properties ofthe PLA polymers.

While specific polylactide polymer compositions for certain applicationsare described below, there are general characteristics desirable for thepolylactide polymer composition. For example, the PLA films of thepresent invention comprise a plurality of polylactide polymer chainshaving a weight-average molecular weight (Mw) of approximately20,000-400,000, preferably 30,000-300,000, and more preferably40,000-200,000. This corresponds to a number-average molecular weight(Mn) of approx. 10,000-200,000, preferably 10,000-100,000.

Polylactide is in equilibrium with its monomer, lactide. This chemicalproperty can lead to rapid hydrolysis and cause problems of adhesion inthe processing of the polymer. Furthermore, the presence of any residualmonomer can lower thermal stability during melt processing. Therefore,residual lactide is typically and preferably removed from the polymer.Preferable monomer content is below about 2%, and more preferably belowabout 1%. Even more preferably, the lactide level is less than about 0.5weight percent or less than about 0.3 weight percent. While it isbelieved desirable to completely remove the lactide from thecomposition, this may not be practical because small amounts of lactidemay re-form. Accordingly, it is expected that the lactide level may beat least above 0.001 percent by weight.

Heat, during film processing steps, can also contribute to polymerdegradation. In this regard, premature hydrolysis of PLA may further bereduced by removing moisture. Accordingly, the water content of thepolymer is preferably maintained below about 500 ppm, and morepreferably, to below about 200 ppm. Methods to further reduce and/ormaintain low water content are described herein below.

PLA is available from multiple suppliers and the polymers and/or polymerblends of the instant invention are not limited to any one grade orsupplier thereof. However, NatureWorks® polymers, as supplied by CargillInc., are preferred in some embodiments of the instant invention (e.g.,grades 4060D, 4042D, 4032D). Each of grades 4060D, 4042D and 4032D has amolecular weight average 200,000 to about 400,000, and the polymerscomprise a mesolactide level of about 10 percent to about 20 percent,and a moisture level lower than about 500 ppm.

Each of the grades differ with respect to the percentages of D-lactideincorporated into the polymer. For example, grade 4042D is prepared withabout 3 to about 5 weight percent D-lactide; grade 4060D comprises about11 to about 13 percent D-lactide; and grade 4032D polymer comprisesabout 1 to about 2 percent D-lactide. These polymers are have a combinedresidual (i.e., unpolymerized) lactide concentration, if any at all, oflower than about 1 percent, and preferably lower than about 0.5 percentby weight. In a preferred embodiment of the present invention, polymergrade 4032D is used.

Copolymers or polymer blends may also be used in PLA films of thepresent invention, but this is by no means necessary for the functioningof the invention. Indeed, in many films, 100 percent by weight of thepolymer component of the composition can be polylactide. However, insome applications, the polymer composition can include other polymers orcomponents blended in with a polymer containing at least 50 percent, byweight, lactic acid residue repeating units. Preferably, the compositionwill include at least about 70 percent by weight polylactide, and morepreferably at least about 90 percent by weight polylactide. It should beappreciated that the amount of polylactide present in a particularcomposition depends on the desired property to be imparted to thatcomposition.

The polymer chosen for blending with polylactide is preferably one whichhas the properties necessary for the particular need and is compatiblewith the PLA. Incompatibility often results in a polymer blend which hasinferior properties, such as very low tensile resistance, rheologicalproperties, degradability, and barrier properties to oxygen, moisture orcarbon dioxide.

Polymers which may be useful for improving the film properties of PLAinclude aliphatic polyesters or polyamides made by both ring opening andcondensation polymerization, esterified cellulose resins, derivitizedstarch, polyvinylacetate and any of its partially hydrolyzed productsincluding polyvinylalcohol, polyethers including poly(ethylene oxide),polycarbonates, polyurethanes including those based on aliphaticisocyanates, polyanhydrides, natural rubber and its derivativesincluding epoxidized natural rubber, block copolymers of styrene andisoprene or butadiene and the hydrogenated version of those polymers,polyacrylates and methacrylates, polyolefins, and polystyrene.

These polymers may be blended with poly(lactide) in percentages of 1 to95 percent by weight to make films of improved properties. Examples ofparticular interest include polymers which are also degradable includingpoly(caprolactone), poly(hydroxybutyrate hydroxyvalerate), celluloseacetate, cellulose acetate butyrate, cellulose acetate propionate, andpoly(vinyl alcohol).

Plasticizers

In accordance with one embodiment of the present invention, theinventive methods described herein enable to manufacture of PLA blownfilms substantially free of plasticizers commonly known, available, andused by one of ordinary skill in the art. Commonly available commercialplasticizers include, for example, mono- and polycarboxylic acid esters,polymeric polyesters, polyalkyl ethers, and glycerol, glycol esters(e.g., glycerol triacetate and glycerol tripropionate), and blends ofthereof. Also various polymeric plasticizers such as citrate esters,adipate esters, epoxidized soy oils, acetylated coconut oil, linseedoil, and mixtures thereof are suitable. As well, it is commonlyappreciated that the addition of lactic acid, lactides (including, D-and L-lactide, meso and racemic D,L-lactide), oligomers of lactic acid,oligomers of lactide, and mixtures thereof, in a PLA blend may act asplasticizers of the polymer.

Alternatively, when desired, the teachings of the present inventionallow for PLA films to be blown with substantially less plasticizer thancommonly used by one of ordinary skill in the art. By way of example,blown PLA films of the present invention may comprise a plasticizer inless than about 2 percent by weight, and more preferably less than about1 percent by weight. When a lactide (or lactic acid) is selected forplasticity, the composition preferably comprises, by weight, less thanabout 10 percent, preferably less than about 5 percent, and morepreferably less than about 2 percent lactide monomers.

Films of the instant invention comprise PLA, and may optionally includeadditives known in the art, e.g., antiblock additives, slip additivesand “viscosity enhancers.” It should be noted that these additives notessential for blowing the PLA films per se, but may be preferentiallyemployed to enhance the processing, performance and look of the finalproduct as will be discussed below. In each of the aforementionedclasses, various grades of the respective polymers are understood to beincluded. Each of these classes of polymers and additives will now bediscussed, in turn, as relevant to the instant invention.

Antiblock Additives

Antiblock (also called “antitack”) additives serve to improve processingand application of polymer films. Specifically, this class of additivesis used to reduce the adhesion between films. Antiblock agents—typicallyfinely divided, solid minerals—act by producing a slight roughening ofthe surface. Antiblock agents are mainly used in film extrusion andinclude natural silica, synthetic silica, talc and calcium carbonate.

Antiblock additives are typically “loaded” with a carrier compound.While it is by no means a requirement, it is preferable that the carrierpolymer be similar to or equivalent to one or all of the polymers in themaster PLA blend. In the instant invention, for example, it is preferredthat the carrier polymer be a PLA polymer. As the “active ingredient” inan antiblock comprises only a small fraction of the final composition,adding a carrier compound provides ease and consistency in measurements.One of ordinary skill in the art would recognize to take theconcentration of filler into account when calculating the finalconcentration of antiblock in the final product. For example, if acomposition comprising 10 percent antiblock consists of 10 percent“active ingredient,” the final concentration of the “active ingredient”is 1.0 percent of the total. The values hereinbelow, including those inTable 1, are provided as a percentage of the “active ingredient” in thefinal formulation.

In the instant invention, the final concentration of antiblock is lessthan about 3 percent by weight, preferably less than about 2 percent byweight, and more preferably less than about 1 percent by weight. In oneembodiment of the present invention, antiblock 2100D from Cargill ispreferred. 2100D comprises 10 percent talc filled magnesium silicate ingrade 4032D PLA carrier and has a mean particle size of less than about1 micron.

In selecting an appropriate antiblock agent, the particle size thereofis one factor to consider. Indeed, the particle size of an antiblockagent can directly contribute to the overall smoothness of the resultingfilm. That is, all other factors being equal, a smaller antiblockparticle size will result in a smoother film. Another factor to consideris the concentration of the antiblock. The concentration of antiblock ispreferably minimized particularly, as in some cases, because antiblockadditives can introduce haze to the film. Hence, particularly inapplications where haze is undesirable (e.g., envelope windows, etc.),the concentration of antiblock may be minimized.

Slip Additives

Slip additives are modifiers that act as an internal lubricant to reducethe coefficient of friction (COF) between two overlapping films, forexample, in films rolled after production. Indeed, lower COFs areespecially desirable for film applications. These additives migrate tothe surface of the plastic during and immediately after processing. Thatis, a non-visible coating “blooms” to the surface to provide amicroscopic “layer” of “lubricant” between two adjacent sheets of film.In this way, enhanced lubricity and slip characteristics are provided.

Accordingly, slip additives may be considered similar to antiblockadditives in that they both serve to lower the COF between twooverlapping films. Films of the instant invention may comprise one, bothor neither class of additives. Typical slip agents are, for example,oleamide, erucamide, stearamide, behenamide, oleyl palmitamide, stearylerucamide, ethylene bis-oleamide, N,N′-Ethylene Bis(Stearamide) (EBS),including most grades of their respective refinement. In someembodiments, EBS is a preferred slip agent, and EBS with 4032D carrieris more preferred. EBS is sold under the tradenames Advawax, Lubrol EA,and Micotomic 280.

As with antiblock agents, the “active ingredient” of slip additives isgenerally supplied with a carrier. Films of the instant inventioncomprise less than about 1 percent by weight of a slip additive(referring to the “active ingredient” only), and more preferably lessthan about 0.5 percent by weight. It should be noted that excessiveamounts of slip additive may produce films that are excessively smooth,which can compromise the ability of substances (e.g., ink, stickers,etc.) to adhere to the surface. Thus, to enhance, for example, theprinting properties of shrink films of the instant invention, the amountof slip additive may require adjustment accordingly.

Viscosity Enhancers

Although numerous methods are known and available to increase theviscosity of polymers during processing of blown films, the term“viscosity enhancer” is defined herein to encompass any chemical agentthat increases or maintains the viscosity of a polymer at a giventemperature. Viscosity enhancers may be introduced into the polymerblend at any time until the polymer enters the die (discussed below),however, viscosity enhancers are preferably introduced prior toextrusion, and more preferably, during blending of the polymer pellets.

Viscosity enhancers can improve the finished properties of films bypreventing and/or reversing the degradation encountered duringprocessing of polymer films. Some viscosity enhancers are “stabilizers.”That is, they are used in virgin plastic to either (1) protect againstdegradation in processing and/or (2) reverse the degradation caused byrecycling, and return the plastic to nearly its original performanceproperties. Another class of viscosity enhancers, “coupling agents,” forexample, improves the processability of extruded polymer by “coupling”individual polymer strands thereby increasing the melt strength of theplastic.

Viscosity enhancers are generally known and available to one of ordinaryskill in the art and have their broadest application with polyesters,polyamides (nylon) and polycarbonates. It is believed that viscosityenhancers have heretofore never been successfully applied to PLA filmtechnology. Though the chemical identity of viscosity enhancers isgenerally proprietary, the products are available from vendors such asJohnson Polymer LLC (USA) and Clariant International Ltd. (Switzerland).Viscosity enhancers suitable in the instant invention are not limited tothose exemplified and/or those with similar mechanisms. In fact, anychemical agent that increases or maintains the viscosity of a polymer ata given temperature may suffice.

In a preferred embodiment of the invention, films are generatedcomprising a coupling agent for enhancing viscosity. Once such couplingagent, CESA®-extend, is available from Clariant. These viscosityenhancers contain a proprietary copolymer of styrene, methylmethacrylate and glycidyl methacrylate. Without being limited by orbound to theory, CESA®-extend visosity enhancers are thought to repairthe damage (e.g., polymer breakdown) that heat and moisture can cause toPLA resins by coupling individual PLA polymers. In this way, theviscosity enhancer may “extend” polymer chains in the presence of somedegradation and thereby attenuate overall loss of molecular weight andviscosity of the polymer melt.

Optimum reactivity of CESA®-extend can be achieved with a preferable uselevel of less than about 0.5 percent by weight, and preferably less thanabout 0.25 percent by weight. Again, the “active ingredient” ofviscosity enhancers is generally supplied with a carrier. CESA®-extendviscosity enhancers may undergo a process residence time of about 2 toabout 20 minutes depending on starting moisture and/or inherent startingviscosity of the polymer. As well, process residence times arepreferably minimized to attenuate polymer degradation from the heatrequired in the process. A detailed discussion of processing conditionsis provided below. In any event, one of ordinary skill in the art wouldappreciate that it may be necessary to adjust the concentration of anypolymer enhancer based, at least, on some of the factors listed above.

In some embodiments, polymers for films may be selected from one, two oreach of four groups (i.e., PLA, antiblock and slip additives, viscosityenhancers) and combined to create blended polymer films. Table 1 belowprovides non-limiting examples of formulations that may be suitable inthe preparation of films of the present invention. For clarity indescription, a skilled artisan should appreciate from the teachingsherein that the percent of additives calculated is only the “activeingredient.” In other words, while it should be appreciated that theadditives are generally supplied in as a combination of “activeingredient” and carrier, the percent of carrier, if any, has not beenlisted. TABLE 1 Polymer Formulations Compositiion (percentage by weight)No. 4060D 4042D 4032D Antiblock Slip Visc. Enhancer 1 69.58%    0%   30%0.15% 0.18% 0.09% 2 97.50%    0%    0% 1.50% 0.50% 0.50% 3    0%    0%99.58% 0.15% 0.18% 0.09% 4    0%    0% 97.50% 1.50% 0.50% 0.50% 5    0%98.58%    0% 0.15% 0.18% 0.09% 6    0%   98%    0% 1.50% 0.50% 0.50% 798.58%    0%    0% 0.15% 0.18% 0.09% 8 59.58%   10%   30% 0.15% 0.18%0.09% 9 49.58%   20%   30% 0.15% 0.18% 0.09% 10 39.58%   30%   30% 0.15%0.18% 0.09% 11 48.61%   30%   20% 1.12% 0.18% 0.09% 12   48%   30%   20%1.00% 0.50% 0.50% 13   30%   20%   48% 1.00% 0.50% 0.50% 14   30%   30%39.58% 0.15% 0.18% 0.09% 15   30%    0% 68.85% 0.15% 0.50% 0.50% 1668.85%    0%   30% 0.15% 0.50% 0.50% 17    0% 68.85%   30% 0.15% 0.50%0.50% 18   30% 68.85%    0% 0.15% 0.50% 0.50% 19 29.58%   30%   40%0.15% 0.18% 0.09% 20   70%    0%   29% 0.50% 0.25% 0.25% 21   29%    0%  70% 0.50% 0.25% 0.25% 22   99%    0%    0% 0.50% 0.25% 0.25% 23    0%  99%    0% 0.50% 0.25% 0.25% 24    0%    0%   99% 0.50% 0.25% 0.25% 25  100%    0%    0%   0%   0%   0% 26    0%   100%    0%   0%   0%   0%27    0%    0%   100%   0%   0%   0% 28    0%    0%    0%  100%   0%  0% 29 99.63%    0%    0% 0.10% 0.18% 0.09% 30    0% 99.63%    0% 0.10%0.18% 0.09% 31    0%    0% 99.63% 0.10% 0.18% 0.09% 32 99.68%    0%   0% 0.05% 0.18% 0.09% 33    0% 99.68%    0% 0.05% 0.18% 0.09% 34    0%   0% 99.68% 0.05% 0.18% 0.09% 35   30%   30% 39.68% 0.05% 0.18% 0.09%36 39.68%   30%   30% 0.05% 0.18% 0.09% 37    0% 39.68%   30% 0.05%0.18% 0.09%

The blends in Table 1 may be chosen or prepared to create the “feel” orflexibility of the film to match an end-use application. Where all otherprocessing parameters are equal, the ratio of PLA polymers may beadjusted in accordance with the teachings of the instant invention toprovide films with desirable physical properties. For example, reducingthe concentration of D-lactide (e.g., increasing weight percent 4032D)yields polymers with a higher softening point.

In a preferred embodiment, films are produced with a blend of about 95.5weight percent 4032D PLA polymer and about 1.5 percent additives byweight. In this embodiment, the additives comprise about 3 percent byweight slip additive and about 1.5 percent of an antiblock.

Processing

Generally, blown films are created by plastic blend being fed into anextruder where it is melted and homogenized before it is pumped througha circular blown film die. See FIG. 1. The melted plastic forms acontinuous tube which is drawn from the die which is inflated by rapidlymoving air. The tube is the collapsed between rollers and wound ontoreels. Alternatively, cast films also begin as plastic dry-blend orgranules fed into an extruder where they too are melted. The differenceis that they are fed through a flat die onto a chilled roller so thatthey cool extremely quickly before being stretched (i.e., oriented) andthen are wound onto a reel.

PLA polymers are typically procured in pellets or grains. In cases wheremultiple polymers are to be included, the polymers pellets are first dryblended. That is, the pellets are mixed together. The processingtherefrom may be reduced to essentially six steps: drying, extrusion,temperature conditioning, orienting, collapsing, and annealing. Eachprocessing step is generally depicted in FIG. 1 and will now bedescribed in detail.

Drying

PLA readily absorbs moisture from the atmosphere and therefore, theblended polymer pellets are preferably first dried by heating in a dryerto remove surface moisture. Without being bound by or limited to theory,it is believed that the removal of moisture content may help control therelative viscosity loss due to hydrolysis. As mentioned above, highertemperatures and the presence of even a small amount of moisture canhydrolyze PLA in the ensuing melt phase.

PLA is generally produced by a reversible condensation reaction, whichproduces water; when undried PLA is heated, hydrolysis can occur and keymechanical properties of the PLA may be compromised. For example, theviscosity of the polymer, when melted, is inversely proportional to thepercentage of free monomer therein. Therefore, in an attempt to minimizebatch-to-batch variation in viscosity, preferably, significant moistureis removed from the polymer pellets. In some embodiments a moisturecontent of less than about 200 ppm is preferable, and less than about 50ppm, more preferable (measured by the Karl Fisher method).

A dehumidifying hopper with hot air at a relatively low dew point may beused; however, a variety of air dryers are known in the art and many ofthem may be suitable for drying. The present invention need not belimited to air dryers only, but may include other types of dryers,including baking ovens. A dehumidifying hopper may be desirable in someembodiments in that dehumidified air passes through a bed of PLA toextract moisture from the resin. A desiccant material, such as silica,absorbs moisture from the circulating air. Dual desiccant bed systemsare common, so that one bed is on-stream while the stand-by bed is beingregenerated. Either a time cycle or a predetermined decrease in air dewpoint is used to shift airflow from one bed to the other. Suchmethodology is thought to be effective in removing some moisture thatmay reside below the surface of the polymer pellets in addition to thesurface moisture.

Preferable dryers of the instant invention for drying PLA may have oneor more of the following characteristics:

-   1. Desiccant beds capable of achieving a dew point of about −40 C in    the supply air-   2. A means, e.g., an after-cooling unit, to eliminate or reduce the    likelihood of temperature spike in the supply air-   3. Excellent temperature control in the PLA drying range

The temperature and duration of drying may be dependent on the totalamount and condition of the polymer(s) (i.e., the amount of startingsurface moisture), and may need to be adjusted on a batch-by-batchbasis. Preferably, the polymers experience little to no melting in thisstep. By way of example, typical drying conditions require thattemperatures range from about 110° F. to about 230° F., and preferablyfrom about 130° F. to about 190° F. for variable periods of time. By wayof example, the residence time for drying polymer with air (dew point,−40° F.) at a flow rate of greater than about 0.5 ft³/min. requiresabout 4 hours at about 110° F. and about 2 hours at about 190° F. Higherdrying temperatures may lead to softening and blocking of polymer, whilelower drying temperatures will result in extended drying times and/orincomplete drying.

Dew point is an absolute measure of air moisture and is independent ofair temperature. Dew point may be used to control dryer performance.Airflow is another component to drying, as it heats the resin andabsorbs its moisture. Sufficient airflow can maintain the resin at theproper temperature for its entire residence time. In embodiments whereadditional colorants, additives, or otherwise ingredients are used, itmay be preferable to minimize moisture-related degradation by furtherdrying same.

Extrusion

Extrusion, the second processing step, is whereby the pellets are meltedinto a low viscosity molten mass, thus combining the heretoforeindividual polymer beads or grains into one molten mass. The viscosityof the melt will depend on the temperature. Temperatures can range fromabout the temperature at which the polymers will remain melted to aboutthe temperature where degradation of the polymers begins to occur. Byway of example, extrusion melt temperatures may be maintained betweenabout 325° F. to about 485° F. for certain PLA polymer blends, but mayultimately depend on the different polymers that have been blended andtheir respective melting points. In some embodiments, about 400° F., ispreferred.

By way of example, the viscosity of PLA at about 480° F. and an apparentsheer rate of about 5.5 seconds⁻¹ in a capillary rheometer may rangefrom about 1,000 poise (P, dyne/cm²) to about 8,000 P, preferably about3,000 P to about 6,000 P, and more preferably, about 4,500 P. At a shearrate of about 55 seconds⁻¹ the same polymer at about 480° F. may have anapparent viscosity that ranges from about 1,000 P to about 5,000 P,preferably about 2,000 P to about 4,000 P, and more preferably, about3,000 P.

Temperature Conditioning

The third step, temperature conditioning, is done to increase theviscosity of the molten polymers, which makes the melt manageable forfurther processing. The cooling allows for the temperature of theextruded polymer to drop to a level at which the corresponding viscosityis high enough to allow a bubble to be blown. Furthermore, it is thoughtthat by increasing the viscosity, a smoother film surface than withoutthis step is generated. A smoother surface aids in the printing processthat is performed in many end applications, such as, for example,labels.

The cooling step may be accomplished by a variety of methods known inthe art, and a variety of coolers are known in the art and may be usedby one of ordinary skill in the art based on the teaching providedherein. For example, the viscosity of the polymer melt may be adjusted,alone or in combination for example, by air cooling the die innermandrel through which the polymer film is blown, the use of viscosityenhancers noted above, controlling the die temperature with air orliquids, or polymer coolers. A Koch Engineering SMR polymer coolingunit, available from Sulzer Chemtech, USA of Tulsa, Okla., adapted forPLA use may be preferred in some applications. By “adapted,” it is meantthat a polymer cooler may have to be adjusted for a cooling capacitylower than that for polystyrene, for example.

The polymer cooler operating temperature range is preferably betweenabout 280° F. to about 450° F. Higher temperatures may be used, but suchhigher temperatures may also contribute to degradation of the polymer.The temperature and duration of cooling can again depend on both theamount of polymer being cooled and the film properties that may bedesired. In other terms, the pressure in the primary loop forpolystyrene cooling is generally about 1000 psi to about 7,000 psi and,in some instances, about 5,000 psi; by contrast, the pressure in thesame loop adjusted for PLA use may range from about 300 psi to about4,000 psi.

In one example, the viscosity of PLA at 375° F. and an apparent sheerrate of about 5.5 seconds⁻¹ in a capillary remoter, may range from about15,000 P to about 17,000 P, preferably about 15,500 P to about 16,500 P,and more preferably, about 16,000 P. At a shear rate of about 55seconds⁻¹ the same polymer at 375° F. may have an apparent viscositythat ranges from about 14,000 P to about 16,000 P, preferably about16,500 P to about 15,500 P, and more preferably, about 15,000 P. It willbe apparent from the data presented herein that the polymer cooling stepcan increase the viscosity from about 2 to about 10 times that of thepolymer coming out of the extruder. In other embodiments, the viscositymay be increased about 5 to about 9 times.

The polymers demonstrate a substantial increase in viscosity uponcooling in the polymer cooler, which cooling procedure, in part, isthought to allow for subsequent blowing of the film. It is also apparentthat the viscosity of the PLA polymers exhibit a consistent shearviscosity of a relatively large range of shear rates at any giventemperature.

Orienting

The fourth step in preparation of films of the present invention isorienting, also known as stretching. Orienting can be accomplished bymany methods and associated equipment known to one of ordinary skill inthe art, including, for example, machine/cross direction orientation andblown film orientation. All methods are preferably designed to firstcontrol the temperature of the polymer, followed by a controlledstretching operation. Without being limited to or bound by theory, it isbelieved that the orienting process conveys strength and flexibility tothe film product. Furthermore, though orientation bubbles may be pulledboth up or down from a die, it may be preferable to pull said bubbleupward to facilitate control and maintenance of the polymer temperatureduring orientation.

In a preferred embodiment of the present invention, the polymer melt isalready pre-cooled, preferably in a polymer cooler, and then submittedto a blown film orientation process. However, the viscosity of thepolymer melt may also be adjusted, alone or in combination for example,by air cooling the die inner mandrel, the use of viscosity enhancers,and liquid thermoregulation of the die. Pre-cooling PLA polymer in apolymer cooler before blown film orienting is believed to be unique. Theprocess of the present invention has at least one significant advantagein that a very controlled temperature—from the post extrusiontemperature conditioning—can be achieved prior to the formation of abubble. A blown film extrusion process extrudes molten plastic polymerthrough a die of circular cross-section and uses an air jet to inflate abubble comprising same.

Die parameters may range from 1:0.75 BUR (Blown Up Ratio) to about 1:7.0BUR, and preferably, about 1:4 BUR in the cross web direction. In thelength (or machine) direction, die parameters may range from about 1:1draw down ratio to about 1:300 draw down ratio, and preferably, about1:130 draw down ratio. Orienting temperatures of the present inventionrange from about 100° F. to about 180° F., and more preferably, about140° F.

In the preferred embodiment then, by virtue of pre-cooling the meltedpolymer, only a final fine tuning of orienting temperature is performed,where desired, during the orientation process. In other words, thegreater share of temperature conditioning takes place prior to orientingand not during orienting. Where a fine tuning of temperature is desired,it can be relatively easily accomplished by a temperature controlled airring, which blows chilled air at the base of the bubble.

Collapsing

Once the extrudate has been inflated into a circular bubble, it then is“collapsed” into a double thickness film. The collapsing process isperformed by use of an “A-frame,” also known as a collapsing frame. Thisframe uses nip rollers, panels, and/or flat sticks to flatten the bubbleinto a sheet of double-thickness film (FIG. 1). The sheets areultimately cut and wound onto two finished rolls, or coils, of PLA film.The sheets of film can also be cut to desired length.

In accordance with another teaching of the present invention, it hasalso been learned that control of the film temperature while in bubbleform may prevent the formation of undesirable wrinkles and/or filmlayers that stick together upon passage through the collapsing niprollers. By control, it is meant that the temperature of the polymerbubble is preferably maintained at a temperature greater than about 100°F., more preferably the temperature is maintained at a range from about100° F. to about 200° F., and even more preferably, from about 120° F.to about 140° F. The temperature of the polymer bubble may be regulatedby a variety of methods, including internal circulation of warm air(e.g., internal bubble cooling (IBC)).

In one embodiment, it has been found that it may be beneficial toinclude a heated oven (FIG. 1) enclosure constructed around thecollapsing frame—and generally extending around a portion of thebubble—to control the temperature at which the bubble is collapsed. Theoven enclosure may optionally extend to and be sealed at or near the topof the bubble to better maintain insulation and temperature control. The“oven” may generally comprise any device that prevents the polymerbubble from cooling below a predetermined temperature, and may includeboth heated panels and/or insulation alone. In a preferred embodiment,the oven comprises a heat source preferably located at or near the topof the collapsing frame. The heat generated therefrom is then maintainedand circulated within the oven by virtue of insulation encompassing thebubble.

Annealing

Annealing, also called crystallization, is the final step in thepreparation of films of the instant invention. Annealing is generallyaccomplished post orienting, and performed at temperatures between about120° F. to about 285° F. in some embodiments.

Exemplar Applications

The compositions and films herein provide environmentally acceptablematerials because their physical deterioration and degradation is morerapid and complete than conventional non-degradable plastics that theycan replace (e.g., polyethylene). That is, the intermediate products ofthe degradation, lactic acid and short polymers thereof, are widelydistributed naturally occurring substances, which are easily metabolizedby a wide variety of organisms to carbon dioxide and water. Thus, theinstant PLA films may a desirable replacement for many conventionalplastic films. Such applications include, but are not limited to,batteries, cans, bottles, disposable lighters, pens and decorativeitems, window boxes for food items (e.g., pastries, donuts), foodscovering (e.g., produce packaging and bags, covering for meats, fish andvegetables), toys, floral bags and window envelopes.

PLA blown films of the present invention may also be used inapplications that have traditionally used paper, for example, envelopesand signage (discussed below). In one embodiment of the invention,envelopes may be entirely constructed with the inventive PLA filmsdescribed herein. Such envelopes may comprise clear, matted or opaquePLA film. As well, in the case of envelopes with windows, the front andrear walls may comprise paper with the envelope window comprising PLAfilm. In yet other embodiments, the front and rear walls of the envelopemay comprise a clear, matted or opaque PLA film and the window comprisea clear or matted PLA film.

As mentioned above, films of the instant invention may be particularlysuited for printing applications. Indeed, the relatively high surfacetension of PLA films makes them receptive to (printed) ink often withoutany additional treatment to the surface. For example, the surface energyof substantially pure polylactide films of the present invention isabout 38 to about 40 dynes/cm. This leads to a surface with satisfactoryprinting characteristics without surface modification. Thus, inks whichare typically more difficult to apply onto films, like water based inks,may be applied directly to PLA films.

The films, printed or otherwise, may also be manufactured with anadhesive on one face to provide pressure sensitive labels. These labelsmay be applied to a variety of consumer products, for example, bottles(e.g., beverages, shampoos, etc.) and general packaging. Otherapplications may include point-of-purchase labels and “piggy-back”labels (e.g., peel-off coupons on consumer products).

It should also be apparent from the teachings disclosed herein that theinstant inventive films are not limited to any one gauge. That is, thethickness of the inventive films may be adjusted in accordance withtechniques generally known and practiced in the art. For example, thetake-off speed may be adjusted to determine the gauge. The “take-off”speed is the rate at which the bubble is pulled though the die;generally, the greater the speed, the thinner the gauge.

It may be desirable to adjust the film gauge to suit a particularapplication. For example, films of the instant invention may be suitableas a paper replacement for indoor and outdoor signage (e.g., plantsticks, tags, folding cartons, etc.). In applications where signage isin proximity to soil, PLA films of the instant invention may beparticularly desirable for their “environmentally friendly”decomposition characteristics.

Pigments, dyes, or color agents may also be added as necessary. Examplesinclude titanium dioxide (TiO₂), clays, calcium carbonate, talc, mica,silica, silicates, iron oxides and hydroxides, carbon black, magnesiumoxide, quinacridone, copper phthaloryanine, ultra marine blue,anthraquinone, pyrazolone, violet B, azo dyes,2,2′-(1,2-ethenediyldi-4,1-phenylene)bisbenzoxazole, and2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole). In one embodimentof the instant invention, white pigment, may be added to the PLA filmsof the instant invention to provide colored or white films.

PLA films of the present invention may also be prepared as “white PLA.”In such embodiments, TiO₂ is a preferable colorant, the concentration ofwhich may range from about 1 to about 20 percent by weight, and ispreferably adjusted to a desired optical density (per thickness offilm). The optical density may be determined in an X-Rite 301 reader.For many applications (e.g., cut and stack labels or pressure sensitivelabels), a density ranging from about 0.40 to about 0.60 absorbanceunits is preferable, with a density of about 0.50 absorbance units beingmore preferable. Antiblock (e.g., talc) may be used to adjust anyinherent gloss to between about 20 and about 100, preferably betweenabout 50 to about 80, as measured at an angle of 60 degrees (ASTM D523).As with other applications described herein, while any grade of PLA(e.g., 4060D, 4042D, 4032D) may be suitable, grade 4032D is preferred.One embodiment of “white PLA” may comprise, for example, by weight, 92percent PLA, 1.5 percent antiblock, 0.20 percent slip additive and 6.3percent TiO₂.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or alterations of the invention following. In general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth and as follows in the scope ofthe appended claims.

1. A method of making a polylactic acid (PLA) blown film comprising (a)providing dry pellets of PLA, (b) melting the pellets to form a moltenmass at a first desired viscosity value or range of values, (c)increasing the viscosity of the molten mass to a second desiredviscosity value or range of values, (d) forming a bubble from theresulting molten mass, and (e) collapsing the bubble to form a film, inwhich the PLA blown film is substantially free of plasticizer.
 2. Themethod of claim 1 in which the pellets are dried in a dehumidifyinghopper.
 3. The method of claim 1 in which the melting step is carriedout at a temperature range of about 325° F. to about 485° F.
 4. Themethod of claim 3 in which the melting step is carried out at atemperature range of about 375° F. to about 425° F.
 5. The method ofclaim 4 in which the melting step is carried out at a temperature ofabout 400° F.
 6. The method of claim 1 in which the first viscosityvalue ranges from about 1,000 P to about 5,000 P at about 480° F. at anapparent shear rate of about 55 seconds⁻¹.
 7. The method of claim 6 inwhich the first viscosity value ranges from about 2,000 P to about 4,000P at about 480° F. at an apparent shear rate of about 55 seconds⁻¹. 8.The method of claim 7 in which the first viscosity value is about 3,000P at about 480° F. at an apparent shear rate of about 55 seconds⁻¹. 9.The method of claim 1 in which the second viscosity value ranges fromabout 14,000 P to about 16,000 P at about 375° F. at an apparent shearrate of about 55 seconds⁻¹.
 10. The method of claim 9 in which thesecond viscosity value ranges from about 15,500 P to about 16,500 P atabout 375° F. at an apparent shear rate of about 55 seconds⁻¹.
 11. Themethod of claim 30 in which the second viscosity value is about 15,000 Pat about 375° F. at an apparent shear rate of about 55 seconds⁻¹. 12.The method of claim 1 in which the viscosity increasing step is carriedout in a polymer cooling unit.
 13. The method of claim 1 in which thestep of forming a heated bubble includes a stretching step, whichorients the film.
 14. The method of claim 13 in which the stretchingstep is carried out using a blown film orientation process.
 15. Themethod of claim 1 which further includes annealing the film.
 16. Themethod claim 15 in which the annealing step is carried out at atemperature ranging from about 120° F. to about 285° F.
 17. The methodof claim 15 in which the annealing step is carried out at a temperatureranging from about 140° F. to about 250° F.
 18. The method of claim 1 inwhich the bubble is heated at a temperature greater than about 100° F.19. The method of claim 18 in which the heating step is carried out at atemperature ranging from about 100° F. to about 175° F.
 20. The methodof claim 19 in which the heating step is carried out at a temperatureranging from about 100° F. to about 160° F.
 21. The method of claim 20in which the heating step is carried out at a temperature ranging fromabout 100° F. to about 140° F.
 22. A method of pretreating a PLA polymerblend to allow the manufacture of a blown film, comprising (a) providingpellets of a PLA polymer blend, (b) melting the pellets to form a moltenmass at a first desired viscosity value or range of values, and (c)cooling the molten mass to a second desired viscosity value or range ofvalues.
 23. A packaged good comprising a consumable item and a PLA blownfilm wrapped around at least a portion of the consumable item, in whichthe PLA blown film is substantially free of plasticizer.
 24. An envelopehaving a front wall and a rear wall, one of said walls being providedwith at least one window, in which the window comprises a blown filmcomprising a polylactic acid polymer and substantially free ofplasticizer.
 25. A blown film comprising a polylactic acid (PLA) polymerand substantially free of plasticizer.
 26. The blown film of claim 25 inwhich the PLA polymer has a molecular weight average of about 100,000 toabout 400,000.
 27. The blown film of claim 25 in which the PLA polymeris greater than about 90 percent by weight polylactide.
 28. The blownfilm of claim 25 in which the blown film further comprises a second PLApolymer.
 29. The blown film of claim 25 in which the PLA polymer has amesolactide level of about 10 percent to about 20 percent.
 30. The blownfilm of claim 25 in which the film further comprises a colorant.
 31. Theblown film of claim 30 in which the colorant is present at aconcentration between about 1 to about 20 percent by weight.
 32. Theblown film of claim 31 in which the colorant is TiO₂.
 33. A blown filmcomprising a polylactic acid (PLA) polymer and less than about 2 weightpercent plasticizer.
 34. A blown film comprising a polylactic acid (PLA)polymer and less than about 10 weight percent of a lactide selected fromthe group consisting of D-lactide, L-lactide, meso D,L-lactide andracemic D,L-lactide.
 35. A blown film according to claim 34 in which theweigh percent of a lactide is less than about 5 percent.
 36. A blownfilm according to claim 35 in which the weigh percent of a lactide isless than about 2 percent.